Dispersal of Late Triassic clam shrimps across Pangea linking northwestern Gondwana and central Pangea rift basins

Clam shrimps are a group of freshwater crustaceans who prospered during the Late Triassic. They were abundant in lacustrine sedimentary records of continental basins distributed throughout Pangea during this time. However, they show significant taxonomic differences between the clamp shrimp faunas from the rift basins of central Pangea and the southern Gondwanan basins. In this contribution, we show new fossil clam shrimp assemblages from the lacustrine sedimentary successions of the Eastern Cordillera of Colombia (the Bocas and Montebel formations), providing information on the Late Triassic species that inhabited the northwestern Gondwana basins. This study demonstrates that the basins of northwestern Gondwana shared Norian clamp shrimp species with rift basins of central Pangea and differed in their faunas with the basins of the southern portion of Gondwana. In addition, the Late Triassic clam shrimps paleobiogeographic distribution reflects the dispersal of this fauna throughout fluvial-lacustrine environments established in the rift valleys along the central Pangea. Therefore, the rift valleys produced during the early fragmentation of central Pangea could have acted as corridors for dispersion. Simultaneously, rift valleys also provided paleobiogeographic barriers that isolated the central Pangea clam shrimp faunas from southern Gondwana.


Geological setting
The Colombian basins in northern South America were located at low paleolatitudes (0º-5º N) at the western margin of Pangea during the Late Triassic 29 .At this paleolatitude, the western margin of Pangea was affected by intracontinental rifting, resulting in a series of extensional basins forming during the Late Triassic [30][31][32][33][34][35] .In the Sierra Nevada de Santa Marta (SNSM) regions and the Eastern Cordillera, the filling of the basins is represented by the Los Indios, Corual, Tiburón, Bocas, Palermo, and Montebel formations.

Biostratigraphy
The assemblage described from the Bocas and Montebel formations in Colombia allows for correlation with the Shipingia hebaozhaiensis zone defined in the Germanic Basin and the Newark Supergroup 22,23,25 .This correlation assigns a middle-late Alaunian age (middle Norian) to the Bocas and Montebel formations 22,23,25 .However, in the middle to upper part of the Bocas Formation, the occurrence of Shipingia olseni, a marker taxa of the Sevatian [21][22][23]25 , in association with Shipingia hebaozhaiensis, indicates the transition between the Alaunian and Sevatian for this specific interval. Th overlap of S. hebaozhaiensis and S. olseni has also been reported in the Newark Supergroup, specifically in the Groveton Member of the Bull Run Formation in the Culpeper Basin 21,22 .

Discussion and conclusions Paleobiogeography
The clam shrimp taxa described from the Bocas and Montebel formations in the Eastern Cordillera of Colombia (Shipingia hebaozhaiensis, Euestheria winterpockensis, Gregoriusella sp., Shipingia olseni, and H. colombianus) www.nature.com/scientificreports/reveal common elements between the northwestern Gondwana basins and the rift basins of central Pangea.The common taxa (Shipingia hebaozhaiensis, Euestheria winterpockensis, Gregoriusella sp., and Shipingia olseni) have previously been reported in the Newark Supergroup for the Culpeper, Gettysburg, Fundy, and Newark basins (U.S.A. and Canada), as well as in the Germanic Basin (Germany).In addition, H. colombianus was reported in the Tinacoa Formation, along the Serranía del Perijá 45 (Venezuela, South America).Therefore, this paleobiogeographical distribution would encompass the rift basins of central Pangea extending to the northwestern Gondwana basins during the middle-late Norian (middle Alaunian-early Sevatian; Fig. 3; Supplementary Table 2).Nevertheless, older reports of clam shrimps preserved in the Los Indios Formation 34 (the Sierra Nevada de Santa Marta, northern Colombia) provide evidence that the distribution of clamp shrimp assemblages in northwestern Gondwana and rift basins of central Pangea could have begun earlier, probably during the Lacian (early Norian).This early distribution is supported by Euestheria ovata Lea, 1856, Euestheria cf.E. hausmanni Schmidt, 1938, Euestheria buravasi Kobayashi, 1975 and Shipingia weemsi Kozur et al., 2012 present in the Los Indios Formation as well as in units of the Newark Supergroup and the Germanic Basin (Fig. 3; Supplementary Table 2).
During the latter, the fractured areas and related subsidences led to the formation of large rift valleys and a network of lakes and aligned river systems 46 .An analogous tectonic scenario can currently be observed in the Great Rift Valley of East Africa, resulting from the fracturing of the African Plate.The tectonic depressions are occupied by elongated and narrow lakes and river systems, reflecting the rift morphology that extends regionally (> 3000 km) from Mozambique to Ethiopia 47,48 .The freshwater environments, supplied by a network of fluvial and lacustrine systems in the rift valleys that developed in a large continentalized Pangea may have provided uniquely favorable habitats for the establishment of the clamp shrimp faunas during the Late Triassic.Additionally, the clamp shrimp reproductive adaptations (i.e.accelerated biological cycle, dioecious reproduction, and the presence of abundant eggs capable of withstanding desiccation 28,[49][50][51][52][53][54] ) could have favored their high abundance in these habitats and the survival resistance of their eggs during transport across rift valleys.
This conditioned dispersion of clam shrimp fauna throughout the central Pangea basins associated with the early fragmentation (rift basins of the central Atlantic margins; Germanic Basin; northwestern Gondwana) could explain the taxonomic difference with the southern Gondwanan clamp shrimp faunas (Supplementary Table 1).Therefore, the rift valleys of central Pangea could have acted as a paleobiogeographic barrier favoring the isolation of these independently evolving faunas (provincialism).

Methods
A total of 330 remarkably well-preserved specimens were collected from the Bocas and Montebel formations for systematic identification.These individuals were obtained from dark lacustrine claystones and siltstones (Fig. 1D).The specimens were observed with a binocular stereo microscope Leica S9D and photographed with a Leica Flexcam C1 camera.The S.E.M. (scanning electron microscope) images were obtained with a JEOL 5800LV microscope at the Secretaría General de Ciencia y Técnica, Universidad Nacional del Nordeste (UNNE).The previously proposed methodology was followed for taxonomic identification and biostratigraphic schemes 18,23,25,55,56 .All specimens are housed in the Instituto y Museo de Ciencias Naturales collection, Universidad Nacional de San Juan (registered numbers PISJ 111-117).

Figure 1 .
Figure 1.(A) The Eastern Cordillera of Colombia in northern South America.(B,C) Detail of the studied areas from the Bocas and Montebel formations.(D) Stratigraphic columns of the Bocas and Montebel formations showing the stratigraphic position of the studied clam shrimps.Maps drawn with QGIS 3.36 (https:// www.qgis.org/ en/ site/ index.html).Stratigraphic columns drawn with Inkscape 1.2.2 (https:// inksc ape.org/).

Figure 3 .
Figure 3. Correlation of stratigraphic units and common clam shrimp species of the Upper Triassic in the basins of northern South America and the rift basins of central Pangea.Species reports and unit ages are based on previous studies 12,21-25,34,45 .Figure drawn with Inkscape 1.2.2 (https:// inksc ape.org/).

Figure 4 .
Figure 4. Paleobiogeographic distribution of common clam shrimp species from the Upper Triassic of the basins of northern South America and the rift basins of central Pangea.(A) Late Tuvalian-early Lacian.(B) Lacian.(C) Alaunian-early Sevatian.The red lines represent the rifting zones of central Pangea during the Late Triassic 2,5,6 .The scheme was developed based on paleogeographic reconstructions for the Late Triassic 57,58 .Figure drawn with Inkscape 1.2.2 (https:// inksc ape.org/).